In particular, don't use goto and do restrict the behavior to darwin.
This addresses comments from http://golang.org/cl/14484.

Change-Id: I5b99e1762d1c5b27fdd12b72a5c6d981f6a92f0f
Reviewed-on: https://go-review.googlesource.com/14673Reviewed-by: Rob Pike <r@golang.org>